Fainting one time is usually nothing to worry about. But it is a good idea to see your doctor, because fainting could have a serious cause.

What causes fainting?

Fainting is caused by a drop in blood flow to the brain. After you lose consciousness and fall or lie down, more blood can flow to your brain so you wake up again.

Most causes of fainting are usually not signs of a more serious illness. In these cases, you faint because of:

The vasovagal reflex, which causes the heart rate to slow and the blood vessels to widen, or dilate. As a result, blood pools in the lower body and less blood goes to the brain. This reflex can be triggered by many things, including stress, pain, fear, coughing, holding your breath, and urinating.

Orthostatic hypotension, or a sudden drop in blood pressure when you change position. This can happen if you stand up too fast, get dehydrated, or take certain medicines, such as ones for high blood pressure.

Fainting caused by the vasovagal reflex is often easy to predict. It happens to some people every time they have to get a shot or they see blood. Some people know they are going to faint because they have symptoms beforehand, such as feeling weak, nauseated, hot, or dizzy. After they wake up, they may feel confused, dizzy, or ill for a while.

When is fainting the sign of a serious problem?

It happens without warning or if it happens when you are already lying down. (When fainting is not serious, a person often knows it is about to happen and may vomit or feel hot or queasy.)

You are losing a lot of blood. This could include internal bleeding that you can't see.

You feel short of breath.

You have chest pain.

You feel like your heart is racing or beating unevenly (palpitations).

It happens along with numbness or tingling on one side of the face or body.

What exams and tests might you need?

To find the cause of fainting, a doctor will do a physical exam and ask questions about the fainting episode. You can help your doctor by being prepared to describe what happened before you fainted, how long you were "out," and how you felt when you woke up.

Depending on what the physical exam shows, the doctor may want to do tests. These tests may include:

Blood tests.

Heart tests such as ECG, ambulatory monitoring (with a Holter monitor or event monitor, for example), echocardiogram, or an exercise stress test.

A tilt table test. This test checks how your body responds to changes in position.

Tests for nervous system problems, such as CT scan of the head, MRI of the brain, or EEG.

What should you do about fainting?

If you know you tend to faint at certain times (such as when you get a shot or have blood drawn), it may help to:

Sit with your head between your knees or lie down if you feel faint or have warning signs such as feeling dizzy, weak, warm, or sick to your stomach.

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